Background: Breast cancer is the most common cancer which disturbs not only the older population but also women under 35 years old. It also ranks to be the first in cancer-connected deaths of women. Objectives: Hence, we intended the study to investigate the efficacy of tomentosin as an anticancer agent against breast cancer in vitro and in vivo conditions. Materials and Methods: Breast cancer was persuaded to the rats by DMBA administration and then treated with the tomentosin for 28 days. The levels of estrogen receptor α (ER-α), oxidative stress markers, biotransformation enzymes, carcinoembryonic antigen, and cytokines were assessed. Histopathological analysis was done to check the anticancer effect of tomentosin. In vitro studies were finished with MCF-7 cells and the cells were exposed to cell viability assay and different fluorescent staining assays such as H2DCFDA, JC-1, and AO/EtBr dual staining to inspect the tomentosin effects. The expression of PI3K/AKT signaling molecules was considered by quantitative polymerase chain reaction (qPCR) analysis. Results: Our in vivo consequences recommend tomentosin knowingly reduced the levels of ER-α, CYP450, CYT-b5, carcinoembryonic antigen, and cytokines and augmented the levels of CAT, GST, and GR enzymes. Histological results authorize anticancer effects of tomentosin. The qPCR results exhibited that tomentosin significantly lessened the expression of PI3K/AKT and augmented the apoptosis-related p38/JNK1 expression in MCF-7 cells. Conclusion: Overall, our results settle tomentosin possibly inhibit the DMBA-convinced breast cancer induction in rats and they also induce apoptosis in estrogen-dependent breast cancer MCF-7 cell lines.